Origins
German 'Stern' and English 'star' descend from the same Proto-Germanic ancestor, *sternô, itself a reflex of one of the oldest and most widespread words in the Indo-European family: PIE *h₂stḗr, meaning simply 'star.' This root is attested in virtually every branch of Indo-European, from Hittite 'ḫasterza' (the oldest attested form, from Anatolia c. 1600 BCE) to modern descendants across Europe and South Asia.
The phonological relationship between 'Stern' and 'star' is instructive for understanding Germanic sound laws. The initial cluster /st-/ is identical in both languages because Grimm's Law — the First Germanic Sound Shift — did not affect voiceless stops when preceded by *s. This is why the /t/ in 'star,' 'Stern,' 'stand/stehen,' 'stone/Stein' remains unchanged from PIE to Germanic, while an isolated *t shifted to *þ (as in PIE *tréyes → English 'three'). The 's + stop' clusters were effectively shielded from the shift.
The vowel difference between German 'Stern' (/ɛ/) and English 'star' (/ɑː/) reflects divergent developments from the Proto-Germanic vowel. Old English had 'steorra' while Old High German had 'sterno' — both show an *e-grade vowel from the Proto-Germanic form. English 'steorra' underwent a complex series of changes: the diphthong 'eo' monophthongized, and the vowel eventually opened to /ɑː/ in early Modern English. German retained the original /ɛ/ quality more conservatively.
Proto-Indo-European Roots
The PIE root *h₂stḗr belongs to a class of r-stem nouns (like *ph₂tḗr 'father,' *meh₂tḗr 'mother') that are among the most archaic formations in the proto-language. The initial *h₂ is a laryngeal consonant — a sound class that had disappeared in all recorded Indo-European languages but whose former presence is deduced from its effects on neighboring vowels and from its preservation in Hittite (as 'ḫ-' in 'ḫasterza').
The cognate set across Indo-European is remarkably rich. Latin 'stēlla' (from an earlier form *stērlā, with dissimilation of the second *r to *l) gave English 'stellar,' 'constellation' (literally 'a group of stars together'), and 'stellate.' Greek 'astḗr' gave English 'asteroid' (star-shaped), 'astronomy' (star-arranging), 'astrology' (star-speaking), and, through Italian, 'disaster' (literally 'ill-starred,' from 'dis-' + 'astro'). Sanskrit 'stár-' appears in Vedic poetry describing the night sky. The Armenian 'astł' and the Tocharian B 'ścirye' further confirm the antiquity of the root.
The semantic stability of *h₂stḗr is notable. Unlike many PIE roots whose meanings drifted considerably across millennia, this word has meant 'star' and nothing but 'star' in every attested descendant for over six thousand years. The celestial referent is unchanging, and so is the word.
Figurative Development
In German, 'Stern' is the foundation of a rich astronomical and metaphorical vocabulary: 'Sternbild' (constellation, literally 'star-picture'), 'Sternschnuppe' (shooting star, literally 'star-snuff' — the star snuffing itself out), 'Sternwarte' (observatory, literally 'star-watch-place'), and 'Sternstunde' (finest hour, literally 'star-hour'). English similarly uses 'star' in compounds both literal and figurative: starlight, starfish, stardom, star-crossed (from Shakespeare's Romeo and Juliet, echoing the same astrological thinking that produced 'disaster').
The persistence of astrological metaphor in both languages — German 'unter einem guten Stern geboren' (born under a good star) and English 'star-crossed,' 'ill-starred,' 'lucky star' — reflects an Indo-European cultural inheritance as old as the word itself. The belief that stars governed human destiny was evidently part of PIE culture, carried by migrating peoples into every corner of the Indo-European world and embedded in the vocabulary of fate and fortune across dozens of languages.